Drill pipe is used in various ways and for different applications such as mining for diamonds, installing public and private utilities, drilling for oil and gas, creating an avenue to link the surface to one or more reservoirs, and linking a location on the surface or the subsurface with another surface or subsurface location. Accordingly, drill pipe comes in specialized configurations particularly adapted for use in one or more different applications. For example, drill pipe may comprise a single wall construction made from exotic steels to withstand hostile fluid and gases. Alternatively, drill pipe may comprise a dual wall construction adapted for use in reverse circulation drilling applications. Depending upon the application and environmental issues, a particular type of drill pipe may be preferable to another based upon cost, proven scientific principles, physical limitations and the like.
Regardless of the application, conventional single-walled drill pipe utilizes the same basic technique: fluids such as drilling muds are pumped down the inside of the pipe and cuttings produced by the drilling process are carried with the drilling mud to the earth's surface along the outside of the drill pipe. More particularly, the cuttings are carried out of the hole either between the borehole and the drill pipe or between a cased hole and the drill pipe. Some exotic types of drilling such as underbalanced drilling deal with the pressure differential between the bottom hole pressures and the surface pressures. This method of drilling is controllable, but it is dangerous.
In addition, single-walled drill pipe exposes the borehole to the drilling mud or fluids until the borehole is cased or cemented. Further, when the returned drilling mud or fluids and cuttings pass through the drilled hole, the hole can become plugged, thereby limiting the movement of the drill pipe. One technique employed to overcome the problem of plugging is to increase the mud flow volume and to circulate the borehole before further drilling is performed. This technique, however, impacts the earth's formation by forming cracks in the borehole, for example. Typically, much, if not all of the additional mud flows into the cracks and/or produces additional cracks. In addition, when the hole is close to the surface, the additional mud can seep or flow to the surface in a process known as “fracing out,” which raises environmental concerns.
Reverse circulation drilling is a distinct drilling technique in which fluids are pumped to the drill bit and cuttings are transferred back to the earth's surface within the drill pipe assembly. This technique can be very advantageous because the drilling mud or fluid has limited exposure to the borehole and creates negligible damming effect. Also, it is environmentally-friendly in drilling applications that involve sensitive aquifers for drinking water and the like. The drill pipe typically used in reverse circulation drilling, however, is very stiff and difficult to steer and bend in a borehole. Thus, its use is limited to relatively straight hole applications, and it is not typically used in deviated hole drilling applications, which are commonly used in the construction, oil and gas, and mining industries.
In conventional drill pipes, wires are typically inserted and spliced inside each drill pipe to communicate with a gyroscope or compass transmitter in order to identify the location of the drill bit below the earth's surface. However, these wires are typically exposed and, therefore, are vulnerable to damage from short circuiting and breakage during the drilling operation.
It would be desirable, therefore, if an apparatus could be provided that would permit double-walled drill string pipe sections to be used for reverse-circulation, horizontal directional and deviated vertical drilling. It would also be desirable if such an apparatus could be provided that would permit the double-walled drill string pipe sections to bend along the arcuate path of a subsurface borehole as freely as a single-walled drill pipe. It would be further desirable if such an apparatus could be provided that would convey larger-sized cuttings and increased volumes of cuttings from the drilling mechanism to the surface of the ground. It would be further desirable if such an apparatus could be provided that would permit drilling in soft, medium or hard rock formations as well as corrosive formations with reduced negative environmental impact and reduced borehole wall damage. It would be further desirable if such an apparatus could be provided that would reduce or eliminate the risk of short circuiting the conductive wires on the drill string pipe sections. It would also be desirable if such an apparatus could be provided that would permit an operator at the ground surface to know immediately what rock or soil formation the drill is cutting as well as the condition of the drill bit. It would be still further desirable if such an apparatus could be provided that would produce a more efficient drilling mechanism by decreasing discharge backpressure experienced during drilling operations utilizing conventional drill pipe. It would be further desirable if such an apparatus could be provided that would achieve longer pilot borehole distances and have a longer lifespan in the borehole. It would be still further desirable if such an apparatus could be provided that would permit the apparatus to be more easily assembled and perform drilling more efficiently, more quickly, and less costly.